This invention relates to a method of separating heavy minerals from a quartz powder and a method of analysing heavy minerals contained in a quartz powder.
In the present context the term “heavy minerals” includes, without limitation, zircon (zirconium silicate), monazites of all kinds (mixed rare earth phosphates), especially xenotime (a mineral containing yttrium phosphate). In addition to rare earth metals they may include low levels of radioactive metals such as thorium and uranium. Such minerals may be present in relatively small quantities (less than 1 ppm) in bulk quartz crystal powders, but they have proved difficult to remove by methods standard in the industry, and even such low levels are undesirable in transparent fused quartz glass products.
Commercial suppliers of high purity quartz powder may produce their powders from a variety of raw materials, and these sources may include pegmatites, i.e. coarse-grained granitic igneous rock. These typically comprise an intimate mixture of feldspars, mica and quartz crystals as well as a diverse range of heavy minerals. By such processes as crushing, leaching, froth flotation etc. a considerable degree of refinement of the quartz is possible, indeed froth flotation is a standard technique in the industry. McEwen et al, “Single-Stage Flotation of Alkali Feldspars, Ilmenite, Rutile, Garnet, and Monazite, with Mixed Cationic/Anionic Collectors”, Society of Mining Engineers, AIME, vol. 260, pp 97-100 (1976), describes the flotation of feldspar and other heavy minerals from quartz using cationic and/or anionic collectors. However, these techniques do not provide complete separation of contaminating species, and many smaller crystals of heavy minerals remain in the quartz powder and are not removed by conventional methods of quartz refinement.
Thus commercially available refined quartz powders have been found to contain heavy mineral particles, typically in the range 5-50 μm in size. Despite the apparently low analytical concentration they exist in large numbers, and are a potential source of defects, some visible with the naked eye and some fluorescing under UV light. In fused quartz glass these particles may dissolve to form a locally contaminated region; they may not dissolve entirely, in which case they may form a local region of phase-separated glass; they may act as nuclei for devitrification of the glass, when typically they may be found at the centre of a roughly spherical region of cristobalite, and finally some of these defects may be found to be associated with a small bubble of gas, which has been liberated by decomposition of the contaminating particle.
Some of these defects would render the glass unsuitable for certain optical applications, and they would also be undesirable in many quartz glass products for the semiconductor industry. When incorporated in a fused quartz crucible as used in the growth of silicon crystal there is the possibility of detachment of a particle leading to contamination of the silicon melt. When present in a fused silica semiconductor jig or window, which may be subject to etching in the course of its use, there is the possibility of detachment of a particle which may be transferred to the surface of the silicon wafer being processed, causing an irreparable defect in a circuit. Where the particle is radioactive, there is the possibility of longer range damage to the semiconductor product. Conventional chemical analysis is an unreliable measure of the content of heavy minerals, many of which are insoluble under normal analytical conditions. Even when chemical analysis is accurate it may not provide a reliable indication of the number of point defects likely to be generated in a transparent glass product.
In gauging the suitability of a given powder for the manufacture of a high quality fused quartz product it is important to be able to assess the number and size of these particulate contaminants, however past analytical techniques have proved to be inadequate.
In mineralogy in general, specific gravity methods have been used for heavy mineral separation. Halogenated hydrocarbon liquids such as bromoform and tetrabromomethane are used in standard glass separating funnels. These methods suffer from a number of disadvantages. Heavy mineral grains are retained on the glass walls of the funnel by electrostatic attraction. The minerals can also be trapped by the quartz grains and prevented from settling. The mineral grains can additionally be entrapped between the stop-cock sliding surfaces. Sealing times in such a separation are prolonged, and for particles in the size range of interest range from many hours to several days, or longer. Finally the liquids which have been used are toxic, and may be absorbed through the skin, or by inhalation of the vapour.